Conventionally, reactive dyes are applied to a fabric by (a) adding the dyestuff to a dyebath containing a fabric, usually cotton, to be dyed, then (b) "salting out" the dye by addition of an electrolyte and then (c) "fixing" the dye on the fabric by the addition of an alkali donor. In this "fixation" process, the pH of the system is adjusted to a level in the alkaline region which promotes reaction between the reactive dye and the cellulosic substrate; see "The Encyclopedia of Organic Chemistry", Kirk Othmer, 3rd Edn., Supplement Vol., 1979, pages 308-310, and "The Dyeing of Cellulosic Fibres", Ed. Clifford Preston, Dyers' Company Publications Trust, 1986, page 4.
The alkali donor most commonly used is sodium carbonate or a mixture thereof with caustic soda; see "The Dyeing of Cellulosic Fibres" (supra).
However, sodium carbonate is a powder which is inconvenient to handle, especially on preparation of a dilute solution for addition to the dyebath or indeed on direct addition to the dyebath (serious dusting can occur), and difficult to store (the powder is prone to caking). Furthermore, on an industrial scale, it is necessary to add large quantities of the powder and this usually involves several additions to the dyebath.
As an alternative, small quantities of caustic soda alone have been added, but careful addition to the dyebath is then necessary, there being a serious danger that the dyer would inadvertently add too much of this and so damage the fabric.
It is also known to use an aqueous solution containing a mixture of sodium silicate and caustic soda in pad dyeing (U.S. Pat. No. 3,843,318) and printing (U.S. Pat. No. 4,092,101) processes. However, at least for some reactive dyes, especially for dyeings applied by immersion in a dyebath, a particularly deep shade is not obtained.
EP-A-0283114 discloses a liquid donor composition containing, as the major ingredient, tripotassium phosphate, together with caustic potash and potassium carbonate. Other alkali donors containing phosphorus compounds such as phosphonates are also known. However, such phosphorus compounds tend to cause environmental difficulties and in certain countries, their use is prohibited.